Injection unit for an injection moulding machine

ABSTRACT

An injection screw is rotationally driven by a rotating motor having a stator and a rotor and can be moved by a number of electrical linear motors, linear motor comprising a primary part, which functions as a stator, and a secondary part, which is linearly movable in the axial direction in a screw cylinder to execute the injection function. The primary parts are assembled to form a first housing-like unit and the secondary parts are assembled to form a second housing-like unit which is coupled to the screw for drive purposes. The rotating motor, fixedly connected to the screw in the axial direction, is arranged with its stator fixedly coupled in the axial direction to the secondary housing-like unit.

PRIORITY CLAIM

This is a U.S. national stage of application No. PCT/DE01/04593, filedon Dec. 03, 2001. Priority is claimed on that application on thefollowing application: Country: Germany, Application No.: 100 61 329.2,Filed: Dec.04, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an injection unit for an injection-moldingmachine for processing plastics or other plasticized materials.

2. Description of the Related Art

Injection-molding machines for processing thermoplastic materials, forexample, have been known for many years. In such an injection-moldingmachine, usually plastics pellets are heated up and melted in a heatedplasticizing cylinder, in which a screw rotates, and the melt issubsequently injected under high pressure into the cavity of aninjection mold. The molten plastics material is then cooled down, sothat it solidifies and is given its final shape. The cooled-downinjection-molded parts are ejected after the mold has been opened. Apartfrom its injection unit, an injection-molding machine also includes aclosing unit for opening and closing the split injection mold. Apartfrom a screw cylinder, the injection unit also includes an injectionnozzle for injecting the molten material into the closed mold. Arrangedin the screw cylinder is a screw, which can be moved back and forth inthe axial direction to execute the injection function, the length of theaxial path of displacement directly determining the amount of materialinjected.

An electric motor is usually used for the rotational drive of the screw,with a mechanical gear mechanism often being interposed. For the axialmovement of the screw during injection, hydraulic drives are mostlyused.

In particular for producing relatively small injection-molded parts,injection-molding machines which have a purely electrical drive havebecome known in recent years. That is to say that the axial longitudinalmovement of the screw is also performed by means of an electric drive.

EP 0 723 848 B1, which corresponds to U.S. Pat. No. 5.679,384. disclosesa number of different embodiments of such fully electricalinjection-molding machines. In one generic type of injection-moldingmachine, it is provided that the screw can be moved together with theelectrical rotational drive unit, which is coupled to the screw, on acarriage, the moving of the carriage taking place by means of anelectromotively driven spindle drive. Consequently, in this case twoelectrical rotational drives are used, one of which uses the spindledrive to convert the rotational movement into a linear movement for theinjecting operation.

Another type of injection-molding machine described in EP 0 723 848 B1provides two electrical hollow-shaft motors lying one behind the other,the first of which has in its rotor a spindle nut for a ballscrew, whichinteracts with a ballscrew connected directly to the screw of theinjection-molding machine, while the second electric motor has on itsrotor a splined-shaft nut, which interacts with a splined shaft whichdirectly adjoins the ballscrew. The length of the splined shaftcorresponds at least to the axial path of displacement of the screw forthe injecting operation. During the return movement of the screw in thematerial-collecting phase of the injection-molding cycle, both motorsare driven at the same speed and in the same direction of rotation. As aresult of the counter-pressure formed by the injection nozzle, in thiscase the screw is pushed linearly backward. Since the splined shaft isin engagement with the splined-shaft nut, in this case the rotationaldrive remains undiminished. During the injection phase, the motor forthe rotational drive is switched off and only the motor with the spindlenut continues to move. Since the screw can no longer rotate, therotation of the spindle nut enforces an axial movement of the screw inthe direction of the injection nozzle and the melt material which hascollected in front of the screw is injected into the injection mold.

Injection-molding machines in which the rotational drive of the screw isgenerated by means of an electrical rotating motor and the lineardisplacement of the screw for the injecting operation is generated by anelectrical linear drive have also already become known. In this case,four linear motors in a parallel arrangement are used, the linear motorsbeing connected to one another to form a cuboidal, housing-like group.The electrical rotational drive and the linear motors are in this casearranged one behind the other in the axial direction. The advantage ofthe electrical linear drive lies in particular in the high acceleratingcapability and, as a result, great injection speed. The comparativelygreat overall axial length of such an injection unit can be seen as adisadvantage.

SUMMARY OF THE INVENTION

The object of the present invention is to improve an injection unit ofthe generic type to the extent that it has a particularly compactoverall length, that is as short as possible in the axial direction,while having a very high injection speed, in particular for producingrelatively small to medium-sized injection-molded parts.

The invention is based on an injection unit for an injection-moldingmachine for processing plastics or other plasticized materials which hasa screw which is provided with a rotational drive by means of a rotatingmotor having a stator and a rotor and can be moved by a number ofelectrical linear motors in each case comprising a primary part, whichfunctions as a stator, and a secondary part, which is linearly movablewith respect to said primary part. In this case, the linear motors areassembled to form a housing-like group with the secondary parts of thelinear motors arranged parallel to one another and the secondary partsof the linear motors coupled to the screw for drive purposes, so thatlinear drive forces can be transferred to the screw. According to theinvention, it is provided that the rotating motor, fixedly connected tothe screw in the axial direction, is arranged within the housing-likegroup and, with its stator, is fixedly coupled in the axial direction tothe secondary parts of the linear motors. In this way, the overall axiallength of the injection unit is distinctly reduced.

In principle, the rotating motor for the rotation of the screw may be ofany desired type, that is for example a hydraulic motor. However,electrical rotating motors, in particular hollow-shaft motors, whichpermit a particularly high drive torque with a low weight, areparticularly preferred for this. The electrical rotating motor isexpediently a synchronous motor.

Particularly preferred is an embodiment in which three linear motors arecombined to form the housing-like group. The three linear motors areexpediently of the same construction, so that the form of an equilateraltriangle is obtained in the axial cross section of the housing-likegroup.

It is advantageous to assemble the secondary parts of the linear motorsby connecting elements to form a self-supporting assembly.

The secondary parts are expediently arranged in each case on a carrierguided in linear guides. This carrier may be respectively formed forexample as a carriage and the linear guides may be respectively formedas sliding rails. For reasons of wear, it is recommendable to supportthe carrier on the linear guides in each case by means of rollingbodies, that is to avoid sliding friction as far as possible. This canalso be advantageously ensured by the carrier being formed as a carriagewith wheels, which roll on the linear guides.

The connecting elements for the assembly of the secondary parts of thelinear motors are advantageously formed as transverse carriers in plateform, arranged at intervals one behind the other in the axial direction.These transverse carriers are arranged perpendicularly in relation tothe longitudinal axis of the linear motors. In this case, it isadvantageous to fasten the electrical rotating motor to the forwardmosttransverse carrier in the direction of injection, that is the transversecarrier lying closest to the injection nozzle. This transverse carrierhas an aperture for the drive shaft of the rotating motor or the screwor a corresponding connecting element (coupling) to pass through.

With regard to the primary parts of the linear motors, it isrecommendable to fasten them in each case to the bottom of across-sectionally U-shaped receiving bed, the ends of the mutuallyparallel side legs of the U form of the receiving bed forming the linearguides for supporting the carriers of the secondary parts. To ensurethat the receiving beds are mechanically held together with respect toone another, the receiving beds may be fixedly connected to one anotherby connecting bars lying against the side legs of the U form from theoutside.

The invention is explained in more detail below on the basis of theexemplary embodiment of an injection unit according to the invention,represented partly schematically in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section through an injection unit accordingto the invention,

FIG. 2 shows the housing-like group of primary parts of the linearmotors from FIG. 1,

FIG. 3 shows the secondary parts of the linear motors according to FIG.1, connected to form a structural unit, and

FIG. 4 shows a perspective view according to FIG. 1.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The injection unit according to the invention, represented in variousviews and sections in FIGS. 1 to 4, has a screw 1, which is connected tothe rotor 4 of an electrical hollow-shaft motor 2 having a stator 3 anda hollow shaft 5. The hollow-shaft motor 2 is connected with itshousing, and consequently also with its stator 3, in a rotationallyfixed manner by means of a transverse carrier 12 a, formed as a plate,to an assembly which is formed by three secondary parts 8 a, b, c, i.e.the movable parts of three electrical linear motors 6. This structuralunit can be seen particularly well in a separate representation in FIG.3. The secondary parts 8 a, b, c are mounted in respective flat carriers11 a, b, c, which have the cross-sectional form of a very flat U. Thecarriers 11 a, b, c are connected to one another by three transversecarriers 12 a, b, c arranged one behind the other at axial intervals. Ascan already be clearly seen from FIG. 1, the hollow-shaft motor 2 isarranged within the volume enclosed by the carriers 11 a, b, c, byflange-mounting on the front transverse carrier 12 a. In order that thescrew 1 or the hollow shaft 5 can be passed through the transversecarrier 12 a to the outside, carrier has an aperture 18 of acorrespondingly large format. It can further be seen from FIG. 3 thatthe carriers 11 a, b, c are provided in the region of their short sidelegs with wheels or rollers 16.

FIG. 2 shows that the primary parts 7, that is the stationary parts ofthe electrical linear motors 6, are assembled to form a housing-likegroup 9, corresponding to the form of the assembly in FIG. 3. Theprimary parts 7, only one of which can be seen in the view chosen, arein each case embedded for this purpose in a receiving bed 17 a, b, c.Since they are formed with the same construction, in principle the threelinear motors 6 form an equilateral triangle in cross section. In a waysimilar to the carriers 11 a, b, c, the receiving beds 17 a, b, c have aU form in cross section, the primary parts 7 in each case being arrangedon the bottom of the U form (on the inner side). Toward the longitudinalside, the primary parts 7 are enclosed by the side legs 14. The ends 13of the side legs 14 form linear guides 10 (FIG. 1), on which the wheels16 of the carriers 11 a, b, c are supported when the assembly accordingto FIG. 3 is fitted into the housing-like group 9 according to FIG. 2,as is represented in the perspective view in FIG. 4. The receiving beds17 a, b, c for the primary parts 7 can also be shaped in such a way thatthey can be directly connected, for example by means of screwconnections, to form an interconnected structural unit. However,connecting bars 15 a, b, c are preferably used for the connections, asrepresented in FIGS. 3, 4. These connecting bars 15 a, b, c are placedagainst the side faces of the legs 14 from the outside and connected tothem in a way not represented, for example by screws. In FIG. 1, it canbe seen that the transverse carriers 12 b, c are also provided withlarge-area apertures, so that the hollow-shaft motor 2 is accessiblefrom the right-hand side.

An electrical connection cable for the primary part 7 of the linearmotor 6, represented in section, is designated by the reference numeral19 b. The connection cable 19 a belongs to the primary part, whichcannot be seen, of the second linear motor.

The operating principle of the injection unit according to the inventionis as follows. During the phase of collecting the melt in the front partof the screw cylinder (not represented), the structural unit mounted onthe wheels 16, with the carriers 11 a, b, c and the secondary parts 8 a,b, c and also the hollow-shaft motor 2, is forcibly displaced to theright as a result of the melt pressure which has built up on theinjection nozzle (not represented). If need be, the screw 1 can besubjected to a moderate forward thrust, which acts to the left withrespect to the representation in FIG. 1, that is in the direction of theinjection nozzle, by the three linear motors 6 to increase thecounter-pressure on the melt. When sufficient melt material hascollected in the screw cylinder, the linear motors 6 are electricallyactivated in such a way that they generate a forward thrust to the leftsufficient for the respectively desired injection pressure, and thestructural unit mounted on the wheels 16 is abruptly displaced with thescrew 1 in the direction of the injection nozzle. As a result, thecollected melt material is injected with the required injection pressureinto the injection mold which is connected to the injection nozzle andis not represented in the figures. The linear motors 6 are formed withlarge surface areas with respect to the cross-sectional area of thescrew 1 and can therefore bring about a considerable injection pressure.Instead of the cross-sectionally triangular arrangement of the linearmotors, which is preferably chosen, if need be it is of course alsopossible to choose another, preferably polygonal form, for example apentagonal form. With respect to the dimensions of the electricalrotating motor located within the arrangement of the linear motors,however, the triangular arrangement already provides a very largeeffective surface area for the linear motors 6. Since no additionalspace is required for the electrical rotating motor, a particularlyshort overall axial length can be realized with the injection unitaccording to the invention.

1. An injection unit for an injection molding machine for processingplastics or other plasticized materials, said injection molding machinecomprising: an injection screw; a rotating motor for rotating saidscrew, said rotating motor being axially fixed with respect to saidscrew and comprising a rotational stator and a rotational rotor locatedinside said stator; and a linear motor arrangement for moving saidrotating motor axially, said linear motor arrangement comprising aplurality of linear motors, each of said linear motors comprising aprimary part and a secondary part, wherein said primary parts areassembled to form a first housing group, and said secondary parts areassembled to form a second housing group which is movable linearlywithin said first housing group, wherein said stator of said rotatingmotor is axially fixed to said second housing group.
 2. An injectionunit as in claim 1 wherein said rotating motor is an electric motor witha hollow shaft to which said rotor is fixed.
 3. An injection unit as inclaim 2 wherein said rotating motor is a synchronous motor.
 4. Aninjection unit as claimed in claim 1 wherein said linear motorarrangement comprises three linear motors.
 5. An injection unit as inclaim 4 wherein said linear motor arrangement has substantially the formof an equilateral triangle in axial cross section.
 6. An injection unitas in claim 1 wherein said second housing group comprises transversemembers to which said secondary parts are assembled to form aself-supporting assembly.
 7. An injection unit as in claim 6 whereinsaid second housing group comprises a plurality of carriers to whichrespective secondary parts are fixed, said first housing groupcomprising a plurality of linear guides in which said carriers areguided linearly.
 8. An injection unit as in claim 7 wherein each saidcarrier is formed as a carriage and each said linear guide is formed asa sliding rail.
 9. An injection unit as in claim 7 further comprisingrolling bodies which support said carriers on said linear guides.
 10. Aninjection unit as in claim 7 wherein each said carrier comprises acarriage with wheels.
 11. An injection unit as in claim 7 wherein saidtransverse members comprise plates arranged at axial intervals, saidplates being arranged perpendicularly to the axis of the linear motor.12. An injection unit as in claim 11 wherein said transverse memberscomprise a forewardmost transverse member closest to said screw andhaving an aperture for said screw, said rotating motor being fixed tosaid forewardmost transverse member.
 13. An injection unit as in claim 7wherein said first housing group comprises a plurality of receiving bedswhich are U-shaped in axial cross-section, each said receiving bedhaving a pair of parallel side legs between which a respective saidprimary part is received, each pair of parallel side legs forming alinear guide.
 14. An injection unit as claimed in claim 13 wherein eachsaid first housing group comprises a plurality of connecting bars, eachsaid connecting bar lying against a pair of side legs of adjacent saidreceiving beds.